Currently, the 3GPP (Third Generation Partnership Project) has proceeded with the standardization of LTE-advanced (hereinafter, specifications of LTE Release 10 and subsequent specifications are collectively called “LTE-A”) that is an evolved radio interface of LTE (Long Term Evolution) Release 8 specifications (hereinafter, referred to as LTE or Rel. 8). LTE-A is aimed at actualizing higher system performance than LTE while keeping backward compatibility with LTE.
In LTE-A, HetNet (Heterogeneous Network) has been studied in which a micro-cell (for example, pica-cell femto-cell, etc.) having a local coverage area with a radius of about several tens of meters is formed in a macro-cell having a wide-range coverage area with a radius of about several kilometers (for example, Non-patent Document 1). In the following description, a radio base station forming the macro-cell is called a macro-base station, and a radio base station forming the micro-cell is called a micro-base station (pico-base station or femto-base station).
A pico-base station that is one of micro-base stations has low transmission power (of about several tens of milliwatts to several watts) and a small cell radius as compared with the macro-base station, but is capable of using radio resources of a bandwidth equal to the macro-base station. Therefore, in order to efficiently accommodate locally concentrated high-density traffic such as a hot spot, offload is studied that the pico-base station accommodates apart of traffic of the macro-base station.
Further, studied is Cell Range Expansion (CRE) that is a technique for providing reception quality (reception power) in cell selection with bias, and thereby increasing the cell radius of the pico-base station. A user terminal existing in the pico-cell expanded by CRE is capable of performing handover (offload) from under the macro-base station to under the pico-base station, and it is thereby possible to increase the offload effect.